This invention relates generally to digital filtering. More particularly, it relates to the design of an efficient sigma delta gain/scaler performing second order filtering with only one sigma delta modulator.
Single bit (i.e., 1-bit) analog-to-digital (A/D) converters which output a single bit value have become very popular, particularly in voice and audio circuit design applications. Single bit A/D converters allow analog circuitry to be relatively simple with high circuit tolerances. Sigma delta techniques are common single bit data processing techniques utilizing oversampling. Generally, oversampling is the digital sampling of an input signal at a much higher rate than a Nyquist minimum rate.
FIG. 1 shows an exemplary single bit A/D converter circuit including a 1-bit A/D converter 3002, a digital decimation filter 3004, and additional post filtering 3006.
In particular, the single bit output of the 1-bit A/D converter 3002 is typically decimated by a digital decimation filter (or simply "decimator") 3004. The decimator 3004 is a type of digital filter which converts a single bit stream occurring at the high (i.e., oversampled) sampling rate into a multi-bit signal (e.g., m bit signal as shown in FIG. 1) occurring, e.g., at the Nyquist rate. Post processing of the low rate, high resolution multi-bit output signal can be performed by post filtering circuitry 3006, e.g., using dedicated hardware filters or a programmable digital signal processor (DSP).
Applications exist in which it is more desirable to process the input signal at the oversampled rate than it is to decimate the single bit signal down to a lower rate, multi-bit signal and then perform post filter signal processing.
For example, FIG. 2 shows a mixing of two separate single bit data streams 3108, 3109 in, e.g., a sigma delta single bit mixer 3104, before being digitized by an appropriate digital-to-analog (D/A) converter 3106.
In particular, in FIG. 2, multiple single bit data streams 3108, 3109 can be mixed together at an oversampled rate in a single bit mixer, e.g., in sigma delta mixer 3104, with the single bit output of the sigma delta mixer 3104 representing a composite of the input single bit data streams 3108, 3109. It may then be desirable to convert this mixed, single bit output stream 3111 into an analog signal 3112 using an appropriate single bit digital-to-analog (D/A) converter 3106.
FIG. 3 shows another application exemplifying the ability to do in-band filtering at an oversampled rate.
In particular, as shown in FIG. 3, early work in this field has shown that it is practical to implement 1.sup.st order filter transfer functions using sigma delta modulators. In this case, it has been shown that a 1.sup.st order filter function was implemented using a classical 2.sup.nd order modulator with a z.sup.-1 delay. A 2.sup.nd order transfer function can be formed by cascading two 1.sup.st order systems, each comprising a sigma delta modulator with a delay of z.sup.-1.
Unfortunately, each sigma delta modulator requires a significant amount of area on an integrated circuit, increasing the size of the overall circuit for each 1.sup.st order block added.
Accordingly, there is a need for a more efficient construction for a 2.sup.nd order digital filter.